add Vagrant Guest Halt capability to vagrant-libvirt. Reference

[Builtin Guest Halt
capability](https://github.com/hashicorp/vagrant/blob/master/lib/vagrant/action/builtin/graceful_halt.rb#L50)

lib/vagrant-libvirt/action/halt_domain.rb

@@ -16,8 +16,14 @@ module VagrantPlugins
           domain = env[:machine].provider.driver.connection.servers.get(env[:machine].id.to_s)
           raise Errors::NoDomainError if domain.nil?
 
-          @logger.info('Trying gracefull shutdown.')
+          begin
+            env[:machine].guest.capability(:halt)
+          rescue
+            @logger.info('Trying libvirt graceful shutdown.')
             domain.shutdown
+          end
+
+
           begin
             domain.wait_for(30) do
               !ready?